A commonly used package for keeping food is a so-called tray package. The tray package is often closed hermetically with a lid. In order to attach the lid to the package, the edge of the package must be equipped with a horizontal rim or edge flange extending from the upper end of the side walls and encircling the package. The upper surface of this flange normally comprises a suitable coating, by means of which the lid can be attached to the package tightly. Examples of food packages of this type, package materials used therein, and methods for forming packages are disclosed, inter alia, in documents WO 03/033258, EP 1 289 856, WO 00/21854, U.S. Pat. No. 5,425,972, WO 03/078012 (corresponding US application 2004/0262322), WO2007/036598 and WO2009/074721.
The above-described tray package is normally formed of a flat blank by pressing it to shape in a mould. The starting material is in a flat shape, that is, extending in one single plane before it is processed by mechanical deforming in the moulding operation to a tray package. Cardboard materials, which are commonly used for such trays, have a sort of memory effect which tend to restore the package tray just formed back to the original flat shape by bending the side walls outwards. Even though the horizontal edge flange or rim stiffens the structure to some extent, there may be slight deformation, called “warping”, in the final package. This shows as a slightly convex shape of the upper surface of the rim.
Apart from being a visual defect, the curved shape of the rim may give technical problems during sealing of the lid to the tray package.
WO2009/074721 describes a tray package which has a plastic flange joined to the upper edge of the side walls of a cardboard tray by injection moulding. The rim made in this way makes the tray rigid and enables easier sealing of the lid. However some plastic materials may shrink a little when solidifying in comparison to the cardboard material, which creates internal forces and consequently curving of the rim.
There is thus a need for a moulded package which overcomes the above mentioned problem with curving of the rim, i.e. warping.